Aging poses the largest risk factor for cardiovascular disease (CVD). Supplementation of vitamin D and/or omega-3 has received attention as a potential anti-aging and cardio-protective strategy. Data from in vitro and animal models suggest that these nutrients are involved in the etiology of aging or behave like a biochemical Fountain of Youth to mediate healthful aging. However, enthusiasm has outpaced the scientific evidence in humans, which is currently based on observational studies and small randomized clinical trials (RCTs) often with small doses. Large clinical trials with a high dose, daily dosing regimen, adequate sample size and trial duration, and longitudinal assessment are urgently needed. The principal function of telomeres is to protect the genome against chromosomal aberrations. Leukocyte telomere length (LTL) is consistently shown to have a direct relationship with longevity, CVD and diabetes in epidemiologic studies, and is thus considered to be a biomarker of aging. What is more, LTL is relatively short in persons with chronic inflammation, which is considered to be part of CVD and diabetic risk. The on-going NIH funded VITamin D and OmegA-3 TriaL (VITAL) is a large, randomized, double-blind, placebo-controlled, 2 x 2 factorial trial of vitamin D3 (2,000 IU/day) and marine omega-3 fatty acid (?-3 FA, 1 g/day) supplements among a representative sample of US men aged ?50 years and women aged ?55 years, with an oversampling of blacks. A unique feature of VITAL is the establishment of a sub-cohort of 1,054 participants who are evaluated in person at the Harvard Clinical and Translational Science Centre (CTSC) in Boston, which allows for in-clinic plasma/buffy coat sample collection and in-depth CVD phenotyping at baseline and Year 2. We propose to examine the effects of dietary vitamin D and/or omega-3 supplementation on LTL attrition assessed longitudinally utilizing in-clinic buffy coat samples collected at baseline, Year 2, and Year 4. Thus, we will also investigate the effects of vitamin D and/or omega-3 supplementation on the longitudinal interrelationship of LTL trajectory, plasma inflammatory cytokines, and CVD risk factors in the VITAL trial. This proposal is in response to the information gaps/research needs identified by the most recent Institute of Medicine (IOM) Report on Dietary Reference Intakes. The IOM report identified the need for randomized clinical trials (RCTs) of vitamin D for prevention of CVD, and cellular biology of vitamin D (e.g. cellular aging) as areas of high priority. The American Heart Association and the IOM recommend that all adults eat fish regularly to reduce risk for CVD. This novel and cost-effective study is expected to provide the information necessary to answer the open question of whether dietary vitamin D and/or omega-3 supplementation are beneficial for LTL shortening. Studies of telomeres will also open new avenues for the basic understanding of vitamin D and/or omega-3 as well as their preventive and therapeutic effects of CVD.